1. Field of the Invention
The present invention relates to a self-sustained pulsating laser diode, and more particularly to a red AlGaInP semiconductor laser used as an optical disk light source in a Digital Versatile Disc (DVD) or the like, and a self-sustained pulsating laser diode in which it is difficult for noise to be induced from light returning from the disc.
2. Related Art
Various low-noise self-sustained pulsating laser diodes with immunity to noise induced by returning light from an optical disc have been reported, such as in the Japanese unexamined patent publication (KOKAI) No. 10-144992 or the Japanese unexamined patent publication (KOKAI) No. 11-220210.
In FIG. 3, one basic technical conception of a self-sustained pulsating laser diode commonly shown in the above-mentioned publication is shown as an example.
In this type of self-sustained pulsating laser diode, a region in the active layer where current is not injected and light is guided functions as a saturable absorber, so that self-sustained pulsating operation occurs.
In a self-sustained pulsating laser diode of the past, as mentioned above and as shown in FIG. 3, it was generally the case that, of the conditions of using a multi-quantum well (MQW) active layer 304 which consists of four or fewer well layers, making a thickness of a flat part of a p-AlGaInP cladding layer 305 to be no greater than 300 nm, making a carrier concentration of a flat part of the p-AlGaInP cladding layer 305 to be higher than 5×1017 cm−3, at least one condition was satisfied.
The reason for this is that, because making the number of well layers of the quantum well active layer 304 small reduces the threshold current density, making a thickness of the flat part of the p-AlGaInP cladding layer 305 thin reduces lateral leakage current (reduces the threshold current and the operating current), and making the carrier density of the flat part of the p-AlGaInP cladding layer 305 high reduces the carrier overflow into the cladding layer from the active layer 304 (reduces the operating current at high temperature).
Note that in FIG. 3, the symbols 301, 302, 303 denote an n-GaAs substrate, an n-GaAs buffer layer and an n-AlGaInP cladding layer, respectively.
While, the symbols 306, 307, 308, 309 denote a p-AlGaInP etching stop layer, a p-AlGaInP cladding layer mesa part, a p-GaInP intermediate layer and a p-GaAs cap layer, respectively.
On the other hand, in FIG. 3, the symbols 310, 311, 312 and 313 denote an n-GaAs current blocking layer, a p-GaAs contact layer, p side electrode a and n side electrode, respectively.
With the structure of the past, for the reasons cited below, there was the problem that self-sustained pulsating operation did not occur at high temperature and high output.
Self-sustained pulsating operation in a self-sustained pulsating laser diode, is caused by a fact that a region in the MQW active layer where current is not injected and light is distributed functions as a saturable absorber.
Therefore, to enhance the self-sustained pulsating operation, it is important to widen the light distribution in the horizontal direction, and to suppress horizontal broadening of current.
To widen the light distribution in the horizontal direction, although it is effective to make an effective refractive index difference parallel to the layers between inside and outside the stripes (Δn) small, with the construction of the past, that is, one of a construction satisfying either or both of the conditions of four or fewer wells of the MQW active layer or a layer thickness in the flat part of the p-AlGaInIP cladding layer of less than 300 nm, it is not possible to achieve a sufficiently small value of Δn, thereby preventing enhanced self-sustained pulsating operation.
If the amount of current injection is large, such as in an operation at high temperatures or in an operation during high-output operation, there is a noticeable broadening of current in the flat part of the p-AlGaInP cladding layer in the horizontal direction.
For this reason, there is an increase in the current distribution compared with the light distribution in the horizontal direction, so that the effect of the saturable absorber is reduced, making it difficult to maintain self-sustained pulsating operation.
With the construction of the past, that is, with a construction having a carrier density of 5×1017 cm−3 in the flat part of the p-AlGaInP cladding layer, the resistance in the cladding layer is small, so that current easily broadens in the horizontal direction, thereby preventing the achievement of sufficient self-sustained pulsating operation at a high temperature or high output.
Accordingly, it is an object of the present invention to provide a semiconductor laser enabling stable self-sustained pulsating operation up to a high temperature.